In electrical power distribution systems, circuit breakers have been used to terminate the flow of current in the event of a fault in the system. Traditionally, in circuit breaker applications, trip units have been thermally based or magnetically based, and perform well for their intended function, which will be appreciated by one of ordinary skill in the art. More recently, interest in microprocessor-based electronic trip units has grown. Circuit breaker trip units having microprocessor controls are being implemented in increasing numbers in power supply circuit breakers. These microprocessor-based trip units replace the traditional thermal and magnetic trip units to initiate automatic circuit interruption by the circuit breaker in response to electrical fault conditions. The popularity of microprocessor-based circuit breakers is largely due to their versatility. That is, a microprocessor-based trip unit provides convenient trip definition and setting adjustment that is precisely tailorable to a particular application.
Typically, a trip unit is mounted within a recess in a circuit breaker enclosure or housing. Current sensing devices within the circuit breaker sense current within the protected circuit. The sensed signal is directed to the trip unit. The trip unit analyzes the sensed signal for one or more indications of fault within the protected circuit. Such indications include: time-delayed-over-current, instantaneous-over-current, over-voltage, under-voltage, over-frequency, under-frequency, over-power, volt-to-current-mismatch, etc. The list of potential fault indications is extensive and within the purview of those skilled in the art to select and design for a given application. If the trip unit detects one of these fault indications, the trip unit provides a single trip signal to a trip actuator within the circuit breaker. The trip actuator actuates an operating mechanism that causes a pair of main current carrying contacts within the circuit breaker to open, thus stopping the flow of electrical current in the protected portion of the distribution circuit.
Typically, signals within a circuit breaker are susceptible to noise and may result in unwanted so-called xe2x80x9cnuisancexe2x80x9d tripping. Noise is unwanted voltage that is induced in electrical circuits and can present a threat to the proper operation of the circuit. Wires and other conductors within a system can pick up stray high-frequency electromagnetic radiation from adjacent conductors in which currents are changing rapidly or from many other sources external to the system. Also, power-line voltage fluctuation is a form of low-frequency noise.
A voltage surge or excessive noise could cause an erroneous signal generated from the microprocessor to propagate a false signal to the trip actuator, thus increasing the frequency of nuisance trips. Additionally, the outputs of a microcontroller may momentarily surge as a result of a power up or the outputs may go to an invalid state when the microprocessor is powering down or only partially powered, potentially causing a nuisance trip in these instances. Such nuisance trips cause unwanted delay and increase component wear unnecessarily. Unfortunately, an improper trip signal due to noise is not detected until a nuisance trip occurs.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by the device and method for verifying a trip unit signal utilizing a trip unit comprising: a sensor for sensing a condition of a circuit providing electricity to a load; a signal converter for converting signals generated by the sensor into a digital value indicative of said condition; a microcontroller for processing the digital value and generating a first signal and a second signal in response to an overcurrent condition; and a logic device configured to process the first signal and second signal and generate a trip signal in response to the first signal and second signal.